1. Field of the Invention
The present invention relates to an electron beam exposure apparatus, more particularly to a technique for measurement of the height of a sample surface in an electron beam exposure apparatus.
2. Description of the Related Art
Advances in microprocessing technology have led to greater integration densities of semiconductor integrated circuits and have resulted in increasingly severe demands on the performance of microprocessing technology. In particular, in exposure, the limit is expected to be reached in the optical exposure technology used in the conventional steppers and the like. Electronic beam exposure is a technique which may well form the basis for the next generation of microprocessing in place of optical exposure.
Electron beam exposure apparatuses include ones of variable aperture (variable size rectangular aperture) exposure, block exposure, and multibeam exposure. Here, the explanation will be given taking as an example the block exposure system, but the present invention is not limited to this. The block exposure system is one in which a pattern of repeating unit graphics is given to a transmission mask, an electron beam is made to pass through it to generate the unit patterns all at once, then these are connected and the repeating unit graphics exposed.
FIG. 1 is a view of the configuration of a beam irradiating system in an electron beam exposure apparatus of the block exposure system. In FIG. 1, reference numeral 11 indicates an electron gun emitting the electron beam, 12 a first convergence lens for converging the electron beam from the electron gun 11 to a parallel beam, 13 an aperture for forming the parallel beam passing through it into a predetermined shape, 14 a second convergence lens for focusing the formed beam, 15 a shaping use deflector, 16 a first mask deflector, 17 a deflector for dynamically correcting astigmatism due to the mask, 18 a second mask deflector, 19 a mask use convergence coil, 20 a first shaping lens, 21 a block mask moved by a stage 21A, 22 a second shaping lens, 23 a third mask deflector, 24 a blanking deflector for controlling the on/off state of the beam, 25 a fourth mask deflector, 26 a third lens, 27 a circular aperture, 28 a reduction lens, 29 a dynamic focus coil, 30 a projection lens, 31 an electromagnetic main deflector, 32 an electrostatic sub deflector, and 33 a reflected electron detector for detecting reflected electrons of the electron beam irradiated on a sample 1 and outputting a reflected electron signal. An electron beam 10 is converged by the projection lens 30 on to a sample (wafer) 1 placed on a stage 2. The stage is made to move two-dimensionally in a plane vertical to the electron beam 10. The above parts are housed in a housing called an electron optical mirror tube (column). The inside of the column is evacuated for the exposure. The electron beam exposure apparatus further has an exposure controller for controlling the parts of the column so as to expose a desired pattern, but the explanation of this will be omitted here.
FIG. 2 is a more detailed view of the configuration of the parts of the main deflector 31 and the sub deflector 32. As shown in FIG. 2, the main deflector 31 is comprised of four electromagnetic deflectors 31a to 31d assembled together. Main deflection data output from a data management circuit 45 is multiplied with deflection efficiencies C1 to C4 at a main deflection first processing circuit 42a to main deflection fourth processing circuit 42d, then converted to analog signals and then amplified at the main deflection first D/A amplifier 41a to main deflection fourth D/A amplifier 41d and supplied to the electromagnetic deflectors 31a to 31d. The electromagnetic deflectors 31a to 31d are made to generate magnetic fields and deflect the electron beam 10 in accordance with the signals supplied to them. For example, as shown in FIG. 3A, one electromagnetic deflector is used to deflect the beam and change its position, then another electromagnetic deflector is used to return it to its original direction thereby enabling the position of emission of the electron beam to be changed, but keeping the direction of emission always vertical to the sample 1. By doing this, even if the height of the sample 1 changes, the exposure position will substantially remain unchanged, so there is the advantage that deterioration of the exposed image can be reduced.
The sub deflector 32 for example is comprised of a ceramic tube on the inner surface of which are formed eight thin metal films extending in the axial direction and serving as electrodes. By supplying voltage to the facing electrodes, an electric field is formed. The incident electron beam is deflected by the electrostatic field. The sub deflection data output from the data management circuit 45 is multiplied with the deflection efficiency D at the sub deflection processing circuit 44, then converted to an analog signal and amplified at the sub deflection D/A amplifier 43 and supplied to the electrodes. Note that for convenience in illustration, only one of each of the sub deflection processing circuit 44 and the sub deflection D/A amplifier 43 is shown, but there are eight electrodes and therefore in actuality eight sets of the sub deflection processing circuits 44 and sub deflection D/A amplifiers 43 are provided corresponding to the electrodes. Deflection efficiencies D1 to D8 are also individually set. As shown in FIG. 3B, the electron beam fired into the sub deflector 32 is gradually deflected and emitted at a certain emission angle.
The deflection efficiencies C1 to C4 and D1 are set so as to give deflection positions proportional to the main deflection data and the sub deflection data given.
In general, the main deflector 31 has a larger deflection range, but a slower response speed compared with the sub deflector 32. Therefore, in the electron beam exposure apparatus, to improve the exposure efficiency, the main deflector 31 and the sub deflector 32 are combined as shown in FIG. 1 and FIG. 2. When performing exposure, as shown in FIG. 4, the deflection range (in actuality a somewhat smaller range) 50 of the main deflector 31 is divided into a plurality of sub regions 51, the deflection position A of the main deflector 31 is made the center of the sub regions 51, and the pattern inside a sub region 51 is exposed while changing the amount of deflection B of the sub deflector 32. Note that the same applies in the case of successively exposing sub regions 51 of the same column while moving the stage.
In the case of an electron beam exposure apparatus used in the process of production of a semiconductor device a semiconductor wafer is used as the sample. A resist is coated on the semiconductor wafer and a pattern is drawn on it by an electron beam. The thickness of the semiconductor wafer is uneven and some warping etc. exists as well. Further, there are changes in height along with movement of the stage. Therefore, there is unevenness in the surface position of the semiconductor wafer placed on the stage 2, that is, the height of the sample. Therefore, it is necessary to measure the height of the sample and adjust the electron beam so that it converges at that height, that is, to adjust the focus position. The focus position of the electron beam exposure apparatus is mainly determined by the projection lens 30, but the focus position can be changed within a small range, but at a faster speed by the dynamic focus coil 29. Therefore, the focus position can be adjusted in accordance with changes in height of the sample by using the dynamic focus coil 29. Note that in an electron beam exposure apparatus, there is the phenomenon called coulomb interaction in which the electrons of the electron beam react with each other resulting in loss of focus of the beam. The focus position changes according to the amount of the electron beam irradiated. The dynamic focus coil 29 is mainly used for the purpose of correcting a change in the focus position, but is also used for adjustment of the focus position in accordance with a change in height of the sample.
The method for measurement of the height of a sample used in the past will be explained next with reference to FIG. 5A to FIG. 5F. A mark 60 is prepared on the surface of the sample 1 in advance using a substance or structure with a different electron reflectance. Further, the reflected electron signal is detected by a reflected electron detector 33 while scanning this mark by an electron beam converged by the projection lens 30. The scanning is performed by changing the amount of deflection of the sub deflector 32 for example. As shown in FIG. 5A, when the focus position is on the surface of the sample 1, the amount of reflected electrons changes rapidly at the edge portion of the mark 60 as shown in FIG. 5B in accordance with the scanning and the signal obtained by differentiation of the waveform exhibits a large peak absolute value as shown in FIG. 5C. As opposed to this, as shown in FIG. 5D, when the focus position is not on the surface of the sample 1, the amount of reflected electrons changes slowly as shown in FIG. 5E according to the scanning and the signal obtained by differentiation of the waveform exhibits a small peak absolute value as shown in FIG. 5F. Therefore, the processing for performing the scanning while changing the focus position near what appears to be the surface of the sample and measuring the amount of the reflected electrons in the above way is performed a minimum of five times and the focus position when the absolute value of the differentiated waveform becomes the largest is made the height of the sample.
When the focus position changes, the deflection efficiencies C1 to C4 and D (D1 to D8) of the main deflector 31 and the sub deflector 32 also change. For example, when the axis of the main deflector 31 or sub deflector 32 becomes inclined in accordance with mounting error, the optical axis also becomes inclined. If the optical axis becomes inclined, the center position becomes offset in accordance with the focus position. This error is designed to be made as small as possible, but cannot be made completely zero. Therefore, the correspondence between the height of the sample and the deflection efficiency, that is, how the deflection efficiencies C1 to C4 and D should be changed when the focus position changes, is found in advance. Therefore, at the time of exposure, the deflection efficiency and the height of the sample are measured at one location serving as a reference on the sample (semiconductor wafer), the height is measured at different points on the sample, the value to be set at the dynamic focus coil 29 is determined for each die (semiconductor chip), and the correction value for the deflection efficiency is calculated from the correspondence and the correction made.
As explained above, in the past, the height of the sample was detected by the method shown in FIG. 5A to FIG. 5F, but this method requires that the focus position be changed and scanning performed at least five times so there was the problem of a long time required for the measurement. If the height of the sample is measured by this method at different points on a sample, a considerable time is required overall.
Further, this method has the problem that since it observes the magnitude of the change in the amount of the reflected electrons at the edge of the mark at the time of scanning by the electron beam, that is, the time differential of the amount of the reflected electrons, so the measurement accuracy is not that good. In particular, the amount of reflected electrons itself is small and it is difficult to improve the accuracy more than that by a mark on an unstable silicon wafer.
Further, when changing the focus position, there is also the problem that axial deviation of the optical system occurs, the electron beam deviates from the mark depending on the shape of the mark, and the focus position is not always achieved when the magnitude of the change of the amount of the reflected electrons becomes maximum.
An object of the present invention is to realize an electron beam exposure apparatus enabling detection of the height of a sample more simply and with a higher accuracy.
To achieve the above object, in the electron beam exposure apparatus and exposure method of the present invention, first and second marks in a predetermined positional relationship are scanned by an electron beam at different incident angles to detect the positions of the first and second marks, and the heights of the first and second marks, that is, the height of the sample, are calculated from the difference between the positional relationship of the first and second marks detected and the predetermined positional relationship and the incident angles.
That is, according to a first aspect of the present invention, there is provided an electron beam exposure apparatus comprised of an electron gun for emitting an electron beam, a converging unit able to converge an electron beam on a sample and make the focus position dynamically move, a deflecting unit for deflecting the electron beam, a movement mechanism for carrying and moving the sample, a deflection data and incident angle relation storing circuit for storing the incident angle of the electron beam on the sample when the electron beam is deflected by the deflecting unit, a mark position detecting unit for detecting a change in reflected electrons at a mark provided on the sample when scanning the mark by the electron beam and thereby detecting the position of the mark, a mark position difference calculating unit for using the mark position detecting unit to scan a first mark provided on the sample by an electron beam of a first incident angle and a second mark in a predetermined positional relationship with the first mark by an electron beam of a second incident angle different from the first incident angle to detect the positions of the first and second marks and calculating the difference in the positional relationship of the first and second marks detected and the predetermined positional relationship, and a height calculating unit for calculating the height of the sample from the difference of the positional relationship of the first and second marks calculated and the relationship of the deflection data and incident angles.
FIG. 6 is a view for explaining the principle of the present invention. Assume that a first mark 60a and a second mark 60b are provided in a predetermined positional relationship on the sample (wafer), for example, exactly a distance L apart from each other. The first mark 60a is scanned by an electron beam 10 of an incident angle xcex1 to detect the position of the first mark 60a, and the second mark 60b is scanned by an electron beam 10 of an incident angle xcex2 to detect the position of the second mark 60b. When at least one of the incident angles xcex1 and xcex2 is not zero, if a mark is not at the focal plane (focus position), the difference in the detected positions of the first and second marks is L, but if a mark is displaced from the focal plane, a deviation xcex94L occurs at the detected position. This deviation xcex94L is determined by the deviation xcex94h of the height of the sample at incident angles xcex1 and xcex2. Therefore, if the difference xcex94L between the detected difference of the positions of the first and second marks and the predetermined distance L is calculated, the deviation xcex94h of the height of the sample can be found from the difference xcex94L and the incident angles xcex1 and xcex2. Therefore, it is necessary that at least one of the incident angles xcex1 and xcex2 not be zero. Further, it is preferable from the viewpoint of computation that one of the incident angles xcex1 and xcex2 be zero or that the incident angles xcex1 and xcex2 be values opposite in sign. For example, if xcex2 is zero, xcex94L=xcex94hxc2x7tan xcex1, while if xcex1=xcex2, xcex94L=2xcex94hxc2x7tan xcex1.
If the deflecting unit has a plurality of deflectors and the amount of deflection is determined by a combination of the amounts of deflection of the deflectors, an electron beam can be deflected to the same deflection position by different incident angles. In this case, it is possible to detect deviation in the height of the sample even by detection of the position of the same mark at the same position.
FIG. 7A and FIG. 7B are views for explaining the principle of measurement of deviation in the height of a sample by scanning the position of the same mark at the same position by an electron beam with different incident angles so as to detect its position. Assume that the deflecting unit is comprised of a first and second deflector. For example, as shown in FIG. 7A, when deflecting from a center O to a position Oxe2x80x2, there is a first combination of an amount of deflection A1 of the first deflector and an amount of deflection B1 of the second deflector and a second combination of an amount of deflection A2 of the first deflector and an amount of deflection B2 of the second defector. As shown in FIG. 7B, the path of the electron beam 10 in the case of the first combination is 10a and the path of the electron beam 10 in the case of the second combination is 10b. The incident angles to the sample differ.
When a sample is at the height shown by Q, the paths 10a and 10b of the two electron beams match at the point Q0 on the sample, but when a sample is at the height shown by P, the paths 10a and 10b of the two electron beams are ones where the beams strike the sample at the positions of P1 and P2, while when the sample is at the height shown by R, the paths 10a and 10b of the two electron beams are ones where the beams strike the sample at the positions of R1 and R2. The difference in the positions of P1 and P2 is determined by the difference h1 of the heights of P and Q and the difference of the incident angles, while the difference in the positions of R1 and R2 is determined by the difference h2 of the heights of Q and R and the difference of the incident angles. The incident angles of the paths 10a and 10b can be calculated from the deflection efficiency and the deflection data and stored in advance in a relation storing circuit. If assuming now that a sample is at a height shown by P, when the mark is detected by the first combination, P1 is detected as the position of the edge of the mark, while when the mark is detected by the second combination, P2 is detected as the position of the edge of the mark, so the height h1 may be found from the difference of P1 and P2 and the incident angles of the path 10a and 10b. The same applies when a sample is at a height shown by R.
As explained above, according to the present invention, since the height of the sample can be measured by scanning by two combinations of different incident angles, the number of scans can be reduced compared with the related art. Further, according to the present invention, since the position of change of the amount of reflected electrons (reflected electron signal) is detected, the measurement accuracy is better compared with the related art where the time differential of the amount of reflected electrons was detected.
Further, since the same mark is detected by scanning by two combinations of different incident angles, the small error in the position of the mark has no effect on measurement accuracy.
Note that if the amounts of deflection A1 and B2 and A2 and B1 are equal, A1 and B1 are opposite in sign, and A2 and B2 are opposite in sign, Oxe2x80x2 matches O and the height of the center position is measured. Normally, when the amounts of deflection are opposite in sign, the incident angles are also opposite in sign, so in this case the paths of the two electron beams become symmetric with the optical axis.
As explained above, the deflecting unit of the electron beam exposure apparatus is a combination of a main deflector having a large deflection range and a sub deflector having a smaller deflection range than the main deflector and performs exposure in the manner shown in FIG. 4. In this case the main deflector corresponds to the first deflector and the sub deflector to the second deflector.
Further, as explained in FIG. 2 and FIG. 3A, the main deflector is comprised of a plurality of electromagnetic deflectors. The direction of progression of the electron beam is changed once, then restored again to its original direction. When deflection is performed by the main deflector, even if the position of incidence to the sample changes, the electron beam is made to strike the sample perpendicularly. Due to this, there is the advantage that even if there is a deviation in the height of a sample, the position at which the electron beam strikes it will not deviate. When detecting deviation in the height of a sample by application of the present invention, the incident angle of the electron beam to the sample is preferably as large as possible. Therefore, when measuring the height of a sample, the incident angle of the electron beam to the sample is made to become larger, while when irradiating the beam in a pattern, the incident angle of the electron beam to the sample is switched to become smaller.
As shown in FIG. 2, the deflecting unit is provided with a plurality of processing circuits able to output deflection data multiplied with coefficients of deflection efficiency and to set the coefficients of deflection efficiency independently and freely for the deflectors and an analog processing circuit receiving the output of the processing circuits and generating drive signals to be supplied to the corresponding deflectors, so in the present invention there is provided an exposure deflection efficiency storing circuit for calculating and storing the combination of coefficients of deflection efficiency for exposure by which the electron beam is deflected by exactly an amount of deflection defined by the deflection data and giving the minimum incident angle of the electron beam to the sample surface and a height measurement deflection efficiency storing circuit for calculating and storing the combination of coefficients of deflection efficiency for height measurement by which the electron beam is deflected by exactly an amount of deflection defined by the deflection data and giving the maximum incident angle of the electron beam to the sample surface. When exposing a pattern on the sample, the deflection is performed by setting coefficients of deflection efficiency for exposure in the plurality of processing circuits. When scanning the first and second marks for detection of the height of a sample, the deflection is performed by setting the coefficients of deflection efficiency for height measurement in the plurality of processing circuits.
The coefficients of deflection efficiency for height measurement are for example all made zero except for one. If only one coefficient of deflection efficiency in the processing circuits of the plurality of deflectors is made a value other than zero and the other coefficients of deflection efficiency of the processing circuits are made zero, return of the direction of the electron beam is not possible, so the incident angle becomes a value other than zero. If just one of the coefficients of deflection efficiency of the plurality of processing circuits is made a value other than zero and the other coefficients are all made zero, computation of the combination of deflection efficiencies for height measurement becomes easy.
Further, it is preferable to correct the deflection efficiency for exposure in accordance with the height of the sample detected. Therefore, provision is made of a deflection efficiency correction value storing circuit for storing in advance the correction data of the deflection efficiency for exposure in accordance with the height of the sample and a deflection efficiency correcting unit for calculating the correction value from the calculated height and the correction data of the deflection efficiency stored in the deflection efficiency correction value storing circuit and correcting the deflection efficiency of the processing circuit at the time of pattern exposure.
In this case, there are the same number of combinations of coefficients of the deflection efficiency where only one coefficient is a value not zero and the other coefficients are all zero as the number of deflectors. It is preferable to evaluate the sharpness of the image of the electron beam on the sample for each combination and make the combination giving the smallest reduction in sharpness among the combinations of coefficients of deflection efficiency giving the same incident angle the combination of the coefficients of deflection efficiency for height measurement. By doing this, the position of the position detecting mark can be accurately detected.
Note that even when determining the combination of coefficients of deflection efficiency for height measurement so that the incident angle becomes a desired value on the condition that a plurality of coefficients being values other than zero, it is preferable to set the combination to give the best sharpness in the same way.
When determining the combination of coefficients of deflection efficiency for exposure, it is determined on the condition that the incident angle of the electron beam on the sample surface always be zero regardless of the amount of deflection and the sharpness of the image of the electron beam on the sample be made the best.